Intestinal Flukes Workup

Updated: Feb 21, 2019
  • Author: Joseph Adrian L Buensalido, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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Laboratory Studies

Stool examination to visualize the ova or adult worms is the diagnostic method of choice.

The eggs of heterophyid species are smaller than eggs of other intestinal flukes. Modified Kato thick method has a higher sensitivity than formalin-ether/ethyl acetate concentration technique (31% vs 13.6%) in detecting heterophyid eggs. [38] Heterophyid species are difficult to distinguish and may also be confused with Clonorchis and Opisthorchis eggs. In areas endemic for liver flukes, heterophyid eggs have been referred to as "opisthorchid-like eggs."

Other laboratory findings include anemia and eosinophilia.

Serologic tests have limited application; however, for certain combinations of pathogens and their available diagnostic testing, serodiagnosis may be helpful, as in the case of F buski infections. [41]

Chronic fascioliasis is generally evaluated with fecal egg counting after concentration of the eggs in the stool sample via a zinc sulphate floatation method. However, using the sedimentation technique to concentrate the eggs is said to improve sensitivity. A F hepatica coproantigen enzyme-linked immunoassay (ELISA) has been introduced and studied in cattle and sheep. It more accurately reflects the presence of flukes in the host bile ducts in late prepatent infections and clearance of the flukes after treatment. [42] It can probably be used in humans in the future.

Urine assay, particularly of O viverrini excretory-secretory (ES) antigens in urine, has been used to detect O viverrini in Thailand. It was found easier to use and more sensitive than the traditional ethyl-acetate concentration technique. [43]

Diagnosing O viverrini infection via conventional stool examination is difficult, both because the infection may decrease in intensity after repeated treatments under control programs in endemic areas and because of the presence of coinfections with intestinal flukes. Thus, one study has examined a coproantigen sandwich ELISA using recombinant O viverrini cathepsin F (rOv-CF) that uses chicken immunoglobulin Y (IgY) raised against rOv-CF in combination with rabbit immunoglobulin G (IgG) antibody to the somatic O viverrini antigens. This test showed a sensitivity and specificity of 93.3% and 76.7%, respectively, in the detection of opisthorchiasis. The investigators found that it had a positive predictive value (PPV) and negative predictive value (NPV) of 66.7% and 95.2%, respectively, making it a promising test in endemic areas. [44]

The current criterion standard of diagnosis is the formalin ethyl-acetate concentration technique (FECT), performed with fecal samples. However, this test has difficulty detecting light O viverrini infections since the eggs may be confused with eggs of other minute intestinal flukes in stool. [43]


Other Tests

Merthiolate, iodine, formalin method

The merthiolate, iodine, formalin (MIF) method is used to detect intestinal fluke parasites.

The MIF method was established early as a versatile and accurate technique for identifying intestinal protozoa in stool and fecal samples. [45] The technique simultaneously preserves and stains stool specimens, which can then be examined with direct smear techniques. [46]

Following the development of a concentrated MIF technique, the sensitivity of positively identifying F buski, Heterophyes species, and Echinostoma species in stool specimens was increased. [47] This newly concentrated MIF technique involves the application of a concentration step to the stool specimen before preservation in MIF solution.

Polymerase chain reaction

Various polymerase chain reaction (PCR) methods have shown potential in detecting intestinal fluke parasites. These methods take advantage of the different types of DNA nucleotide sequence variations demonstrated by the different species of parasites within a particular genus. [48]

In Thailand, Lamanignao et al developed a PCR-based technique to detect and discriminate between O viverrini and Haplorchis taichui. O viverrini and heterophyids have similar egg morphology, so an experienced microscopist is usually required for distinction. Both parasites have similar intermediate and definite hosts and may exist as coinfections. [49] More importantly, chronic O viverrini infection is associated with the development of cholangiocarcinoma. [50] Although both can be treated with praziquantel, Opisthorchis infection requires two days of treatment.

Likewise, the difficulty of diagnosing parasitic pathogens led Won et al to develop a multiplex qPCR to detect the presence of eight major intestinal parasites known to cause gastroenteritis in the Korean population. This includes three trematodes (C sinensis, M yokogawai, G seoi). Analysis included 123 stool samples, with the multiplex qPCR exhibiting 100% sensitivity (95% CI; 80.5-100) and 100% specificity (95% CI; 96.58-100). [51]

Polymerase chain reaction–restriction fragment length polymorphism (PCR-RFLP) [17] and simple sequence repeat anchored PCR [52] have been reported to be useful in distinguishing among species of the Metagonimus genus (including M yokogawai). These methodologies are based on differences in restriction fragment length polymorphisms and simple sequence repeats among the species. Information derived from RFLP involving specific sites in ribosomal RNA and mitochondrial cytochrome oxidase I (mtCOI) genes may help to differentiate M yokogawai from other Metagonimus species. [53]

Six species of the genus Heterophyidae were reported to be distinguished with PCR assays developed based on variations in rDNA polymorphisms among the species. [54]